Nanoparticles can have properties intermediate between molecular and bulk forms of matter. For example, nanocrystals based on semiconductor materials having small diameters can exhibit quantum confinement of both the electron and hole in all three dimensions, which leads to an increase in the effective band gap of the material with decreasing crystallite size. Colloidal gold nanoparticles, are a suspension (or colloid) of sub-micrometer-sized particles of gold in a fluid, usually water. The nanoparticles can have a variety of shapes, including spheres, rods, cubes, and other shapes. Generally, gold nanoparticles are produced in a liquid by reduction of chloroauric acid, HAuCl4. Galvanic reactions where a substrate acts as the reducing agent can produce colloidal Au nanoparticles that can be well controlled for size, but generally with little or no control over nanoparticle location on a substrate.
Bulk synthesis of semiconductor nanowires can be achieved using several variations of transition metal catalyzed techniques such as vapor-liquid-solid (VLS) synthesis. In standard VLS synthesis a nanowire grows from a single particle of a metallic catalyst, e.g., gold, cobalt, nickel or other metal. Nanowire precursors are then exposed to the catalyst particles. Typically this occurs at elevated temperatures, where the precursors are in the gas phase.